Android applications pulled in nearly as much mobile advertising revenue as iOS apps in May, according to Millennial Media, one of the largest remaining independent mobile ad networks. This excludes revenue earned from sales of virtual currency or offers, an area where iOS still shines relative to Android as developers tell us they’re still making six to 10 times as much on Apple’s platform as Google’s.

Millennial says Android now makes up more than half of the smartphone impressions it serves — almost double what iOS has.

“We expect Android to start to steal share from Apple as developers start to create more applications for the Android platform to take advantage of the high volume of customers who have shifted to Android devices,” the company said.

Windows Phone is unsurprisingly the fastest growing platform in terms of smartphone impressions, probably because it’s growing from the lowest base. Millennial says Windows Phone 7 impressions are growing 92 percent month-over-month while the iPad is growing at a 29 percent month-over-month rate.

In terms of device breakdown, Millennial says Motorola’s Droid phones are still the most popular Android devices, followed by the HTC Evo, then the Droid X, the HUAWEI Ascend, and then the Samsung Galaxy S.